The present invention relates to an improvement in the structure of a filter which functions with acoustic waves running close to the surface in a substrate wherein digital structures are provided as reflectors and/or input/output transducers.
A filter corresponding to the invention can be operated not only with acoustic surface waves (Rayleigh and Bleustein waves) in the narrowest sense but also with Lamb waves, Love waves, surface-skimming bulk waves and the like which proceed in a substrate at least close to the surface. The type of wave generated in the individual case depends on techniques in dimensioning the transducers which are known to a person skilled in the art, and may also depend on anisotropy properties of the substrate.
Surface wave resonator filters in the sense of the invention comprise digital structures situated on a piezo electric substrate, these digital structures including interdigital structures to be employed as transducers and reflector structures. Interdigital structures are formed of strip electrodes adjacent to one another and electrically connected at sides of the structures. The digital structure of a reflector is formed of strip-like fingers, finger pieces, dots or the like which are preferably metallization strips applied to the substrate surface.
An arrangement and dimensioning of the fingers, strips, and the like of the structures are based on the rules for the respective filter. The fundamentals for dimensioning and measurement of the digital structures are known. Let German OS No. 29 09 705, U.S. Pat. No. 4,325,037, and German OS No. 3 314 725 be referenced in this regard.
Bus bars are provided for interdigital structures, the fingers of the respective one finger structure being connected to one another with these bus bars. The fingers of the one finger structure engage in comb-like fashion into a corresponding, second finger structure and form the interdigital structure with the latter. A terminal pad which is usually relatively large in area connects to these bus bars situated to the side of the interdigital structure. The leads required for the interdigital structure employed as a transducer are connected to these pads which are connected to the respective bus bar, or form a part thereof.
The quality and performance of such a surface wave filter depend, among other things, on the exact fashioning, positioning, and precise, sharp-edged limitation of the strip-shaped fingers of the respective digital structure whether this is a matter of an interdigital structure or of a reflector structure.